1. Field of the Invention
The present invention relates to a wavelength-division-multiplex signal transmission system, and more particularly to an improvement in such a wavelength-division-multiplex signal transmission system for increasing reliability in the event of a failure of a transponder.
2. Description of the Related Art
Research and development activities have intensively been made in recent years with respect to wavelength-division-multiplex signal transmission systems for dealing with a large amount of information. One wavelength-division-multiplex signal transmission system employs transponders.
A transponder is a device for converting the wavelength of an optical transmission device into a wavelength suitable for wavelength-division-multiplex transmission. An existing optical transmission device and a transponder may be combined into a wavelength-division-multiplex signal transmission system for increasing the transmission capacity of conventional systems.
FIG. 1 of the accompanying drawings shows a conventional wavelength-division-multiplex signal transmission system which employs transponders.
As shown in FIG. 1, the conventional wavelength-division-multiplex signal transmission system generally comprises a transmitting system and a receiving system.
The transmitting system comprises terminal units 1-1.about.1-n, transmitting transponders 2-1.about.2-n, a wavelength-division-multiplexer 8, an optical fiber amplifier 9, and an optical cogmbiner/divider (hereinafter referred to as a "WDM coupler") 17-1, and a supervising (SV) signal transmitting unit 6. The receiving system comprises a WDM coupler 17-2, an optical fiber amplifier 9, a wavelength-division-demultiplexer 10, receiving transponders 13-1.about.13-n, and terminal units 16-1.about.16-n, and a supervising (SV) signal receiving unit 15.
In the transmitting system, the transmitting transponders 2-1.about.2-n convert the wavelengths of optical signals transmitted from the respective terminal units 1-1.about.1-n into respective wavelengths .lambda.1.about..lambda.n suitable for wavelength-division-multiplex transmission. The optical signals having the respective wavelengths .lambda.1.about..lambda.n which are outputted from the transmitting transponders 2-1.about.2-n are wavelength-division-multiplexed by the wavelength-division-multiplexer 8, amplified by the optical fiber amplifier 9, and then sent to the WDM coupler 17-1. The WDM coupler 17-1 combines the wavelength-division-multiplexed signals amplified by the optical fiber amplifier 9 with an SV signal outputted from the SV signal transmitting unit 6, producing a combined optical signal. The combined optical signal is applied to and transmitted through an optical fiber transmission path 18 to the receiving system.
In the receiving system, the combined optical signal transmitted from the transmitting system is divided into the wavelength-division-multiplexed signals and the SV signal by the WDM coupler 17-2. The SV signal is supplied to the SV signal receiving unit 15, and the wavelength-division-multiplexed signals are supplied to the optical fiber amplifier 9. The wavelength-division-multiplexed signals are amplified to a desired level by the optical fiber amplifiers, and then separated into the optical signals having the respective wavelengths .lambda.1.about..lambda.n by the wavelength-division-demultiplexer 10. The wavelengths .lambda.1.about..lambda.n are then converted into suitable wavelengths by the receiving transponders 13-1.about.13-n. The optical signals having the respective suitable wavelengths are supplied from the receiving transponders 13-1.about.13-n to the terminals units 16-1.about.16-n.
Even if the wavelengths of the optical signals transmitted from the respective terminal units 1-1.about.1-n are the same as each other, since the wavelengths are converted by the transponders 2-1.about.2-n, the optical signals can be wavelength-division-multiplexed, and hence the capacity of transmission by the wavelength-division-multiplex signal transmission system is relatively large.
Since the wavelengths that can be outputted from the respective transponders are fixed, auxiliary transponders need to be added for the respective wavelengths in order to provide against transponder failures. Furthermore, when a certain transponder, e.g., the transponder 2-1, suffers a fault and needs to be replaced with an auxiliary transponder which produces the same wavelength as the transponder 2-1, the terminal unit 1-1 must be switched off. Consequently, the wavelength-division-multiplex signal transmission system has a reliability problem in the event of a transponder failure.